1. Field of the Invention
The invention relates generally to thermal analysis and, more particularly, to differential thermal analysis performed by using a scanning calorimeter.
2. Description of the Relevant Art
Differential thermal analysis is an old and well-known method for the analysis of materials. Basically, the method consists of simultaneously applying heat to a sample material and a reference material. As the sample material goes through various physical and chemical changes, such as crystallization, melting, freezing, oxidation, etc., its temperature is affected by the changes in internal energy. The differences in temperature between the sample and reference are recorded and, from this data, calculations may be made for determing the internal energy changes occurring in the sample.
One such method, and the apparatus to implement it, is disclosed in U.S. Pat. No. 3,263,484 issued Aug. 2, 1966 to Watson, et al, assigned to the Perkin-Elmer Corporation of Norwalk, Conn. U.S. Pat. No. 3,263,484 is hereby incorporated by reference.
The referenced patent teaches both apparatus and methods for performing dynamic power difference calorimetry, where; (a) a sample and a reference are subjected to a predetermined temperature program; (b) the heating powers supplied to the sample and the reference are varied, one related to the other, as a function of the temperature differences between sample and reference to eliminate the temperature differences; and (c) the difference of these heating powers, supplied to the sample and the reference, is then provided as output.
U.S. Pat. No. 3,263,484 goes on to teach that when a sample is subjected to a temperature program, for example a temperature increased linearly with respect to time, endothermic or exothermic transformations may ocur in such a sample whereby energy is consumed or energy is supplied. This may concern physical transformations such as melting, but also, for example, chemical reactions.
The determnation of the points at which these transformations occur may give information about the type of sample being analyzed. In order to measure this transformation heat, in the arrangement of the prior art, a sample and a reference are each arranged on a heatable plate such that heating energy may be supplied to the sample and the reference separately and independently of each other. One temperature sensor is arranged at the sample and another at the reference.
A program control makes sure that the average value of the temperatures of sample and reference follows a predetermined temperature program. Furthermore, a control system is provided which eliminates the temperature difference between sample and reference by asymmetrically supplying heating energy to the sample and the reference.
A measuring device is provided, which converts the difference of the heating powers supplied to the sample and to the reference, into a voltage difference, which is then recorded as a measure of the transformation heats of the sample.
Similar methods and apparatus are described in "Analytical Chemistry", 36 (1964), page 1233 to 1238, and in "Analytical Chemistry 26 (1964), page 1238 to 1245.
Further yet, a method for quantitative thermal analysis with a differential calorimeter is described in "Chemie-Technik" 5 (1976) page 321 to 325. In this arrangement a sample and a reference are also arranged in two different separately heatable furnaces. A program pickup controls, through a program amplifier, the heaters of the furnaces for the sample and reference. A temperature sensor which responds to the temperature of the sample and of the reference, respectively, is arranged at each of the two furnaces. The differences of these two temperatures control through a "T amplifier", a heating power amplifier which is arranged to apply compensating heat to the sample and the reference such that the temperature difference is eliminated. A recorder uses, as abscissa, the temperature predetermined by the temperature pickup, and as ordinate, the compensating heat supplied by the heating power amplifier.
The aforesaid methods and devices of the prior art require two furnaces. An extremely high technical expenditure is required for adjusting the two furnaces exactly enough such that an exactly reproducible relation is ensured between the difference of the heating powers supplied to the sample and the reference, and the real energy consumption (or supply) of the sample. The two furnaces with the associated adjusting and controlling means constitute a further considerable expenditure in construction cost.
Accordingly, it would be desirable to have apparatus and methods for performing power compensation in a differential scanning calorimeter which eliminates the problems associated with the prior art.